Return (instancetype) instead of (id)

Constructors should generally return `instancetype` rather than `id`. 

See following URLs for details on instancetype.

- http://nshipster.com/instancetype/
- http://stackoverflow.com/a/16743857/2672573
- https://github.com/github/objective-c-conventions/pull/26

**Use case**
Use as a model the generated model subclass.

- https://github.com/azu/AcceleratorWish